On 12.04, running do-release-upgrade -d will offer you an upgrade to 14.04, the most recent LTS. Running it on 14.04 will offer you an upgrade to the the current development release codenamed utopic unicorn which will become Ubuntu 14.10.

Your current Hardware Enablement Stack (HWE) is going out of support
on 08/07/14. After this date security updates for critical parts (kernel
and graphics stack) of your system will no longer be available.